FIGS. 1 to 3 of the accompanying drawings show the design of known heat exchangers. FIG. 1 is a vertical cross section through the heat exchanger while FIGS. 2 and 3 show alternative tube face cross sections taken in the plane II-II in FIG. 1. The heat exchanger comprises a shell 10 and a core 12. The core has two end plates 14, 16 which define headers 18, 20 at the top and the bottom of the shell 10. A set of tubes 22 is welded or expanded or both in holes in the two end plates 14, 16 to define fluid flow passages between the two headers and baffle plates 24, 26 support the tubes 22 along their length and maintain the spacing between them.
The tubes 22 can be disposed in a square pitch array, as shown in FIG. 2, with a typical spacing of 10 mm or less or in a triangular pitch array, or as shown in FIG. 3, with a typical spacing of 10 mm or less, the latter allowing a greater concentration of tubes.
In use, a first fluid is pumped via inlets and outlets and 30 to flow through the tubes 22 and a second fluid is pumped via connectors 32 and 34 to flow through the shell 10. The tubes are made of a good thermal conductor, so that a transfer of heat takes place between the two fluids during their passage through the heat exchanger.
Prolonged flow of fluids through the shell and the tubes can result in the formation of deposits and a reduction in the efficiency of the heat exchanger. It is, therefore, essential at intervals to clean the heat exchangers to remove such deposits.
The present invention is concerned only with the cleaning of a shell side of the tubes 22. To do this in the prior art, the entire core needs to be separated from the shell so that access can be gained to the external, i.e., shell side, surfaces of the tubes 22.
The conventional way of cleaning the shell side of the core is to use high pressure jetting. Narrow jets of fluid emitted from the front end of a handheld lance are aimed at the outermost surfaces of the tube nest to be cleaned to dislodge deposits adhering to the outer surfaces of the tubes. The fluid is usually water at between 1000 psi and 40,000 psi, but for certain applications it may be preferred to use other liquids or gases as the cleaning medium.
Such a lance is referred to herein as a handheld lance, to distinguish it from known lances, such as that shown in European Patent Application EP 0307961, that are mechanically fed in through a hole in the shell wall and are used to clean the header and the baffle plates. A handheld lance is one that is capable of being handheld and moved along the length of the core tubes by an operator. The term “handheld” is not intended to preclude the possibility of such a lance being mounted on a mechanical arm to permit automation of the cleaning process.
Conventional handheld lances include a conduit about 10 mm in outer diameter with a jet nozzle at its tip. Because of its large outer diameter, when cleaning a core of the type shown in FIG. 3, a conventional lance cannot be inserted between the tubes of the core and the high pressure jetting is carried out with the nozzle outside the core in the hope that the water will penetrate between the tubes and remove the deposit from scaled tubes. In the case of the core of FIG. 2, a lance can be inserted into the two wider slots provided for this purpose and the lance may be provided with lateral nozzles but the lance cannot be inserted between all the tubes of the heat exchanger.
The effectiveness of a high pressure fluid jet decreases as the distance from the nozzle to the surface being cleaning increases. For this reason, when using a large diameter lance, only the visibly accessible outer tubes near to the outside of the core can be cleaned efficiently.
It is, therefore, desirable to form a lance of tubing having a smaller outer diameter to be capable of being manually inserted between all the tubes of a heat exchanger. However, a long lance of narrow diameter would be incapable of withstanding the reaction force of a high power jet and would tend to buckle. It would be unsafe to use such a lance because the high power water jet, if uncontrolled, is capable of causing serious injury to the operator.